Hree-component reaction of benzylamines, diethyl phosphite and triethyl orthoformate: Dependence of the reaction course on the structural features of the substrates and reaction conditions

Article abstract of DOI:10.3390/molecules22030450

The reaction between benzyl amines, triethyl orthoformate, and diethyl phosphite affords either bisphosphonic (compound 1) or N-benzylaminobenzylphosphonic (compound 2) acid depending on the reaction conditions. The final output of the reaction can be manipulated by the choice of reaction conditions, particularly the molar ratio of substrates.

Full text of DOI:10.3390/molecules22030450

molecules
Article
Three-Component Reaction of Benzylamines, Diethyl
Phosphite and Triethyl Orthoformate: Dependence of
the Reaction Course on the Structural Features of the
Substrates and Reaction Conditions
Patrycja Miszczyk 1, Ilona Turowska-Tyrk 2, Paweł Kafarski 1 and Ewa Chmielewska 1,
*
1
Department of Bioorganic Chemistry, Faculty of Chemistry, Wrocław University of Science and Technology,
50-370 Wrocław, Poland; patrycja.miszczyk@pwr.edu.pl (P.M.); pawel.kafarski@pwr.edu.pl (P.K.)
The Advanced Materials Engineering and Modelling Group, Faculty of Chemistry,
2
˙
Wrocław University of Science and Technology, Wybrzeze Wyspian´skiego 27, 50-370 Wrocław, Poland;
ilona.turowska-tyrk@pwr.edu.pl
*
Correspondence: ewa.chmielewska@pwr.edu.pl; Tel.: +48-71-320-2977
Academic Editor: Margaret A. Brimble
Received: 23 February 2017; Accepted: 9 March 2017; Published: 11 March 2017
Abstract: The reaction between benzyl amines, triethyl orthoformate, and diethyl phosphite
affords either bisphosphonic (compound 1) or N-benzylaminobenzylphosphonic (compound 2)
acid depending on the reaction conditions. The final output of the reaction can be manipulated by
the choice of reaction conditions, particularly the molar ratio of substrates.
Keywords: bisphosphonates; aminophosphonates; N-benzylaminomethylenebisphosphonic acid;
N-benzylaminobenzylphosphonates; synthesis of C-P bond; multicomponent reactions
1. Introduction
Bisphosphonates are a prominent class of organophosphorus compounds. They are most
commonly used to treat postmenopausal- and glucocorticoid-induced osteoporosis by inhibiting
the mineralization or resorption of bone [
1–
4]. Recently, they have also been used as drugs to
treat bone problems accompanying from multiple myeloma [
5,
6
] and as palliative agents [ ].
7,8
Aminomethylenebisphosphonates are a subclass of bisphosphonates, and representatives from this
group are promising inhibitors of physiologically important metalloenzymes, such as glutamine
synthetase (a target for tuberculosis) [9], hexokinase (a promising agent against protozoan species
that cause African sleeping sickness and Chagas disease) [10], undecaprenyl diphosphate synthase
(a potential antibacterial agent) [11,12], HIV-1 integrase and reverse transcriptase [13,14].
Developed by Maier [15] and Kantoci et al. [16], a three-component reaction between amines,
triethyl orthoformate and diethyl phosphite (Scheme 1) is perhaps the simplest and most commonly
used procedure for the preparation of N-substituted aminomethylenebisphosphonic acids [17,18].
This reaction typically yields a complex mixture of products which are not separated but
immediately hydrolysed with concentrated hydrochloric acid to provide the desired N-substituted
aminomethylenebisphosphonic acids in moderate to good yields.
P(O)(OH)2
P(O)(OH)2
1./
+
HP(O)(OEt)2
R NH2
H-C(OEt)3
+
R
2./ HCl/H2O
N
H
Scheme 1. Three-component reaction of amines, triethyl orthoformate and diethyl phosphite.
Molecules 2017, 22, 450; doi:10.3390/molecules22030450
www.mdpi.com/journal/molecules

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This reaction is also quite unpredictable and frequently affords unexpected products [17
,
18], and
the composition of which depends on the applied conditions (molar ratio of substrates, temperature
and reaction time). The complexity of this reaction could be partially explained by its mechanism.
Studies using 31P-NMR and the isolation of all intermediates have revealed that the reaction proceeds
by the formation of four intermediates which are in thermodynamic equilibrium [19].
When using benzylamine as a substrate, we have observed that, depending on the
reaction conditions, either N-benzyl aminomethylenebisphosphonic acid (
aminobenzylphosphonic acid ( ) were formed (Scheme 2). Here, we describe this reaction in some
detail using structurally variable benzylamines as substrates.
1) or N-benzyl
2
+
H-C(OEt)3
+
HP(O)(OEt)2
H
NH2
N
P(O)(OH)2
S
S
H2O/HCl
4
P(O)(OH)2
P(O)(OH)2
H
N
+
H2N
P(O)(OH)2
P(O)(OH)2
+
N
P(O)(OH)2
H
3
1
2
Scheme 2. Reaction of benzylamine with triethyl orthoformate and diethyl phosphite.
2. Results and Discussion
2.1. Benzylamine as a Substrate
The products obtained in the reaction of benzylamine with triethyl orthoformate and diethyl
phosphite are indicated in Scheme 2.
If the reagents were used in a 1:2:1 stoichiometric ratio (benzylamine:phosphite:orthoformate)
under standard literature conditions, mixtures of the desired N-benzyl aminomethylenebisphosphonic
acid (
1
) and N-benzyl aminobenzylphosphonic acid (
2
) were obtained (Table 1 entries 2 and 3).
◦
The reaction carried out at lower temperature (80 C, entry 1) provided compound
2
as the sole
product in good yield. The structure of this compound was additionally confirmed by its independent
synthesis via the addition of diethyl phosphite to a Schiff base obtained from benzylamine and
benzaldehyde [20]. Microwave stimulation also produced compound 2 as the sole reaction product,
albeit in low yields (entries 4–7). Prolonging the reaction time to over 12 min resulted in complex
mixtures of products which were difficult to separate.
Table 1. Products of the reactions between benzylamine, triethyl orthoformate and diethyl phosphite.
Amine: Phosphite:
Molar Amounts of
Products (1:2) *
Total Isolated
Yield (%)
Entry
Orthoformate
Molar Ratio
1:2:1
Reaction Conditions
◦
1
2
3
4
5
6
7
8
80 C, 12 h
0:100
43:57
65:35
53
10.5
20
NR
11.5
5
◦
1:2:1
1:2:1
1:2:1
1:2:1
1:2:1
1:2:1
1:4:2
135 C, 12 h
reflux with removal of ethanol/12 h
◦
80 C, 6 min, MW
◦
80 C, 12 min, MW
0:100
0:100
0:100
100:0
◦
135 C, 6 min, MW
◦
135 C, 12 min, MW
13.5
68
◦
135 C, 12 h
* determined basing on 31P-NMR spectra of separated products.

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It is worth mentioning that the use of 2-thiophenemethylamine as a substrate under standard
◦
conditions (1:2:1 molar ratio of substrates, 135 C) also provided the corresponding aminophosphonic
acid (compound 4, Scheme 2) in satisfactory yield.
The formation of compounds
2 and 3 in a reaction designed to prepare the corresponding
aminomethylenebisphosphonates was surprising and difficult to explain based on the known
mechanism of the three-component reaction (for more detailed discussion see next paragraphs).
However, the desired bisphosphonate
1 was obtained in good yield when applying the reagents
in a molar ratio of 1:4:2 (entry 8). Thus, there is the possibility to steer the course of this reaction by
manipulating with ratios of substrates, which prompted us to study the behaviour of benzylamines
with substituents in the aromatic ring.
In some cases, the formation of minute quantities of aminomethylenebisphosphonic acid (3) was
also observed (this compound is clearly seen in the 1H-NMR spectra as a triplet at approximately
1.5 ppm with JPH = 16.5 Hz [21]). Because this compound was previously prepared by the acid
hydrolysis of N-benzhydrylaminomethylenebisphosphonic acid [22], we speculated that it might
also be formed upon the hydrolysis of compound 1. However, this did not appear to be the case,
as prolonged refluxing of the latter in boiling concentrated hydrochloric acid failed to produce
compound 3.
2.2. Reactions of Benzylamines with Substituents on the Phenyl Ring
First, reactions were carried out under conditions that provided mixtures of products
5 and 6
◦
(1:2:1 molar ratio, 135 C, 12 h of heating). In most cases, aminophosphonates
6 were obtained as
single products (Table 2, entries 3, 7–10, 14), or they predominated the reaction mixtures (entries 12, 13).
Additionally, the formation of minute quantities of compound
its appearance was quite random.
3 was sometimes observed. However,
The substituents on the phenyl ring showed significant effects when studying the series of
benzylamines bearing a methyl substituent on the phenyl ring. Thus, for m-methylbenzylamine,
aminophosphonate 6b was obtained as the sole product of the reaction in molar ratio 1:2:1 (entry 3),
whereas in the case of p-methylbenzylamine (in the same molar ratio of the reagents), a mixture
of products was formed with bisphosp◦honate 5c being the predominant one (entry 5). Lowering
the temperature of the reaction to 80 C resulted in the formation of aminophosphonate 6c as
the sole product (entry 7). Interestingly, for o-methylbenzyl amine (entry 1), the formation of an
additional product
7 was observed. The same results and formation of products 16 were observed
for o-chlorobenzylamine used as a substrate. This compound is the hydrolysed form of one of
intermediates that was identified earlier [19]. The possible route to its formation is shown in Scheme 3.
Although the combined yield of compounds 6a and
as a significant fraction, as the molar ratio of 6a:7 was only 73:25 (based on the 31P-NMR spectrum).
Since the formation of aminophosphonic acids is somewhat surprising, we succeeded in isolating
7 was low (10%), this side product was obtained
6
one of them, compound 6d (entry 8), in crystalline form, and thus, its structure was unequivocally
confirmed by X-ray crystallography. The asymmetric unit contains two nitrogen-protonated species,
one of which possess a singly deprotonated phosphonate group (thus, is zwitterionic), while the
second one is fully protonated with the charge neutralized by a chloride anion (Figure 1).
In the reaction with o-methylbenzylamine as a substrate, the change in the molar ratio of reagents
to 1:4:2 once more resulted in a change of the reaction course, and aminomethylenebisphosphonate
5a was obtained in satisfactory yields (entry 2). The only exception was the reaction with
p-chlorobenzylamine (entry 11), which afforded a mixture of products. Thus, steering the reaction
course by manipulating the reaction conditions is also effective for other benzylamines.

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Table 2. Products of the reactions between substituted benzylamines, triethyl orthoformate
and diethylphosphite.
X
X
X
+
+
H
N
H-C(OEt)3
HP(O)(OEt)2
PO3H2
PO3H2
NH2
+
N
H
PO3H2
5
X
6
Substituent
(X)
Amine:Phosphite:Orthoformate Molar Amounts of
Total Isolated
Yield (%)
Entry
Product(s)
Molar Ratio
Products (5:6) *
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
o-CH3
o-CH3
m-CH3
m-CH3
p-CH3
p-CH3
p-CH3
o-Cl
m-Cl
p-Cl
p-Cl
p-F
6a
5a
6b
5b
6c
5c
5c + 6c
6d
6e
6f
5f + 6f
5g + 6g
5h + 6h
5h
1:2:1
1:4:2
1:2:1
1:4:2
1:2:1
1:4:2
**
100:0
0:100
100:0
85:15
100:0
0:100
0:100
0:100
0:100
57:43
26:74
36:64
100:0
31
19
36
7
28
13
24
31
24
75
53
33
88
NR
◦
1:2:1 (80 C)
1:2:1
1:2:1
1:2:1
1:4:2
1:2:1
1:2:1
1:4:2
1:2:1
m-Br
m-Br
o-NO2
6i
* determined basing on 31P-NMR spectra of separated products. ** Additional product
NR—no reaction products detected.
7
was observed (see text).
OEt
+
+
H-C(OEt)3
N
NH2
NH2
- OEt
OEt
-2 OEt
1. HP(O)(OEt)2
2. HCl/H2O
H
H
H
N
N
N
N
P(O)(OH)2
7
Scheme 3. Presumed route to the formation of compound 7.
Figure 1. The crystal structure of molecule 6d
. Displacement ellipsoids are drawn at a 20%
probability level.

Molecules 2017, 22, 450
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◦
The reaction of
α
-methylbenzylamine (135 C, 1:2:1 molar ratio) gave a nearly equimolar mixture
-methylbenzylamino)methylenebisphosphonic acid (compound , Scheme 4) and
aminomethylenebisphosphonic acid (compound ). Changing the molar ratio of the reagents to 1:4:2
of the desired N-(
α
8
3
resulted in the production of compound 8 as the sole reaction product.
H2O/HCl
H2N
PO3H2
PO3H2
H
+
H-C(OEt)3
+
HP(O)(OEt)2
N
PO3H2
+
NH2
PO3H2
3
8
Scheme 4. Reaction of α-methylbenzylamine.
Different courses for the reaction were obtained when using aminobenzylamines as substrates.
Aminomethylenebisphosphonic acid (compound ) was the major product in the reaction (85%
3
yield) of p-aminobenzylamine. The formation of this product may be explained by the formation of
the intermediate imine 11 (suggested by mass spectrometry of the crude reaction mixture), which
decomposes to compound
the reactions of o- and m-aminobenzylamines provided complex mixtures of compounds with the
expected bisphosphonates and their N-ethylated derivatives 10 (Scheme 5) It is worth mentioning
that ethylation has been previously found as an infrequent side reaction accompanying the studied
3
(Scheme 5). As deduced from the 1H-NMR spectra of the crude products,
9
three-component reaction [18
from crude reaction mixture.
,19
,23]. The expected bisphosphonates
9
were isolated by crystallization
H2N
H2N
NH2
NH2
1. HC(OEt)3/HP(O)(OEt)2
2. HCl/H2O
H
N
H
H
H2N
N
P(O)(OH)2
P(O)(OH)2
N
P(O)(OH)2
P(O)(OH)2
H
HN
+
N
P(O)(OH)2
P(O)(OH)2
9a (o-) or 9b (m-)
10
H
N
P(O)(OH)2
N
P(O)(OH)2
P(O)(OH)2
H2N
P(O)(OH)2
11
3
Scheme 5. Reaction with the aminobenzylamines as substrates.
2.3. Presumable Mechanism of Reaction Leading to N-Benzyl Aminobenzylphosphonic Acid (2)
Most likely, compound is produced by the addition of diethyl phosphite to
2
N-benzylidenebenzylamine 12. The latter compound might be formed by a process already described
in the literature, in which benzylamines undergo catalytic oxidative dehydrogenation yielding the
appropriate Schiff bases [24,25]. Compound 12 is the presumed substrate for the production of
compound via the simple addition of phosphite [20]. Another possibility is the formation of
2
benzaldehyde in the reaction of benzylamine with triethyl orthoformate followed by a process similar
to the transamination–transimination (TATI) reaction [26]. The formed benzaldehyde then reacts
with benzylamine yielding N-benzylidenebenzylamine 12 (Scheme 6). Taking into consideration all

Molecules 2017, 22, 450
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the observation and accounts given above, a mechanism of the whole process of aminophosphonate
formation could be proposed (Scheme 6).
TATI
+
H-C(OEt)3
N
OEt
NH2
N
OEt
H
H
H
[O]
+
O
N
NH2
12
1. HP(O)(OEt)2
2. HCl/H2O
H
N
P(O)(OH)2
2
Scheme 6. Possible route for the formation of N-benzylidenebenzylamine 12.
Thus, we decided to study the reaction of benzylamine with triethyl orthoformate (2:1 molar
ratio) under anaerobic (under argon) and oxidative conditions (atmospheric conditions and conditions
resulting from the addition of a small quantity of hydrogen peroxide to the reaction mixtures). Quite
interestingly the reaction of benzylamine and triethyl orthoformate carried out under argon gave
nearly exclusively N,N’-dibenzylimino formamide 13 as the product, the reaction under atmospheric
conditions gave a complex mixture of compounds with compounds 12 and 13 as the major products,
and the reaction carried out under the presence of hydrogen peroxide gave N,N’-dibenzylmethanol
(compound 14, Scheme 7), which was identified by spectroscopic methods and its conversion to the
O-acetyl derivative. N,N’-Dibenzylmethanol did not react with diethyl phosphite at all.
HC(OEt)3/air
N
+
H
N
N
NH2
12
13
HC(OEt)3/H2O2
AcOCl
HC(OEt)3/argon
H
H
N
H
N
H
H
N
N
O
OH
N
N
O
14
13
Scheme 7. Reaction of benzylamine with triethyl orthoformate under anaerobic and aerobic conditions.
The influence of the presence of oxygen on the course of the three-component reaction was also
studied under standard conditions, namely, by heating the reagents at 135 ◦C for 12 h. Despite the
applied conditions (under argon or by applying small quantities of hydrogen peroxide), reactions
carried out using a stoichiometric ratio of the substrates (1:2:1) gave practically equimolar mixtures
of products (aminomethylenebisphosphonate
(Similary to that shown on Scheme 5). The presence of significant quantities of hydrogen peroxide
in the reaction medium resulted in the formation of N-benzyl aminobenzylphosphonic acid (
1 and N-ethylated aminomethylenebisphosphonate
2
)

Molecules 2017, 22, 450
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in high (95%) yield. This supports the speculation that the oxidation of imines has a significant
influence on the course of the reaction. It is also worth noting that the processes carried out
under oxidative conditions additionally resulted in elevated ethylation of the formed phosphonates.
Finally, reaction of o-chlorobenzylamine with triethyl orthoformate, provided the derivative of
N,N’-di(o-chlorobenzyl)iminol formamide 15 obtained as the major product with satisfactory yield
(31% yield). Reaction of this compound with diethyl phosphite (1:2 molar ratio) followed by the
acid hydrolysis of the reaction mixture provided two products (30% total yield; Scheme 8), namely,
amino phosphonate 6d and a side-product that was not isolated from the reaction mixture. Based on
the spectroscopic and MS data of the mixture of these products, the structure of 16 was tentatively
proposed for this compound. Reaction carried out using a stoichiometric ratio of the substrates
(1:2:1) gave aminomethylenephosphonic acid (compound 6d) as the major product in the reaction of
o-chlorobenzylamine (Table 1 entry 8).
Cl
Cl
HC(OEt)3
130OC, 6h
Cl
H
N
N
NH2
15
1./ HP(O)(OEt)2
2./ H2O/HCl
Cl
Cl
Cl Cl
+
H
H
N
N
N
PO3H2
PO3H2
6d
16
Scheme 8. Reaction of N,N’-di(o-chlorobenzyl)imino formamide with diethyl phosphite.
3. Experimental Section
3.1. General Information
All solvents and reagents were purchased from commercial suppliers, were of analytical grade
and were used without further purification. Unless otherwise specified, solvents were removed with a
rotary evaporator. The 1H-, 31P- and 13C-NMR spectroscopic experiments were performed on a Bruker
Avance II Ultrashield Plus (Bruker, Rheinstetten, Germany) operating at 600.58 MHz (1H), 243.12 MHz
(31P{1H}) and 151.016 MHz (13C), or on Bruker Avance III 500 MH (Bruker, Rheinstetten, Germany)
operating at 500.14 MHz (1H) and on Jeol JNM-ECZ 400S Research FT NMR Spectrometer (JEOL Ltd.,
Tokyo, Japan) operating at 399.78 MHz (1H), and 161.98 MHz (31P{1H}). Measurements were made in
DMSO-d6, CDCl3 and solutions of D2O + NaOD at 300 K, and all solvents were supplied by ARMAR
AG (Dottingen, Switzerland). Chemical shifts are reported in ppm relative to TMS and 85% H3PO4,
used as external standards, and coupling constants are reported in Hz. Melting points were determined
on an SRS Melting Point Apparatus OptiMelt MPA 100 (Stanford Research Systems, Sunnyvale, CA,
USA) and are reported uncorrected. Mass spectra were recorded at the Faculty of Chemistry, Wroclaw
University of Science and Technology using a Waters LCT Premier XE mass spectrometer (method of
electrospray ionization, ESI) (Waters, Milford, MA, USA).
3.2. Crystallography
The single-crystal X-ray data were collected with a KM4 CCD four-circle diffractometer. The cell
constant determination, data collection and data reduction were performed using the CrysAlis
PRO software package [27]. The structure was solved and refined by the SHELXS-2013/1 and

Molecules 2017, 22, 450
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SHELXL-2014/7 programs, respectively [28
,
29]. All non-hydrogen atoms were refined with anisotropic
displacement parameters. All hydrogen atoms were located in a difference Fourier map and
refined freely. The structure was visualized with the Ortep-3 program [30]. The experimental
and crystallographic data are included in the cif file deposited at the Cambridge Crystallographic
Data Centre: CCDC 1409625. Crystal data. C28H29Cl5N2O6P2, M = ◦728.72, triclinic, a = 9.2663(4),
b = 12.6722(6), c = 14.5066(7) Å,
α = 98.233(4), β= 105.849(4), γ
= 94.450(4) , V = 1609.55(13) Å3, T = 299 K,
space group = P1, Z = 2, 20921 reflections measured, and 6340 unique reflections (Rint = 0.0254) which
were used in all calculations. The final wR2(F2) and R1(F) were 0.0970 (for all data) and 0.0389 (for
5454 observed data), respectively.
3.3. General Procedure for the Synthesis
A mixture of an amine (0.03 mol) and the appropriate amounts (see tables) of dieth◦yl phosphite
◦
and triethyl orthoformate were either heated with stirring at an elevated temperature (80 C or 135 C)
for 12 h on the heating plate of a Radley’s Carousel or placed in a microwave synthesizer (Biotage
Initiator + SP Vave) for the time indicated in Table 1. Then, the volatile components of the reaction
mixture were evaporated, and the resulting crude product was hydrolysed under reflux for 12 h with
6 N hydrochloric acid (20 mL). After evaporation, the crude products were obtained and purified by
crystallization from water or water-ethanol mixtures if necessary.
◦
N-Benzylaminomethylenebisphosphonic acid (
1
) was obtained as a white solid (68% yield); m.p. 231–234 C
(lit. m.p. = 246–249 C [31]); 31P-NMR (243.12 MHz, D2O + NaOD, ppm):
(600.58 MHz, D2O, ppm): = 2.61 (1H, t, J = 17.56 Hz, C P), 3.84 (2H, s, C 2), 7.19 (1H, t, J = 6.9 Hz,
Ar), 7.27 (2H, t, J = 7.1 Hz, Ar), 7.32 (2H, d, J = 7.1 Hz, Ar); 13C-NMR (151.02 MHz, D2O + NaOD, ppm):
= 54.21, 59.42 (t, J = 125.8 Hz,
P), 127.62, 128.74, 128.89, 138.51; HRMS (TOF MS ESI−): [M H]−
Calcd for C8H12NO6P2: 280.0140, found: 280.0150.
δ = 17.26; H-NMR
◦
1
δ
H
H
δ
C
−
N-Benzylaminob◦enzylphosphonic acid (
2
) was obtained as a white solid (53% yield); m.p. 227–228 ◦C (lit.
= 15.89; 1H-NMR (600.58 MHz,
P), 7.10–7.20
H]− Calcd for
m.p. = 236–237 C [20]); 31P-NMR (243.12 MHz, D2O + NaOD, ppm):
δ
D2O, ppm):
δ = 3.35 & 3.50 (2H, AB system, J = 11.2 Hz, CH2), 3.56 (1H, d, J = 19.4 Hz, CH
(4H, m, Ar), 7.21–7.26 (4H, m, Ar), 7.27–7.32 (2H, m, Ar); HRMS (TOF MS ESI−): [M
C14H16NO3P: 276.0789, found: 276.0784.
−
N-(2-Methylbenzylamino)methylenebisphosphonic acid (5a) was obtained as a white solid (31% yield); m.p.
◦
◦
235–236 C decomp. (lit. m.p. = 256–257 C [31]); 31P-NMR (243.12 MHz, D2O + NaOD, ppm):
1H-NMR (600.58 MHz, D2O + NaOD, ppm):
= 2.03 (s, 3H, C 3), 2.43 (t, 1H, J = 16.9 Hz, C
& 7.01 (AB system, 4H, J = 7.6 Hz, Ar); 13C-NMR (151.02 MHz, D2O + NaOD, ppm):
59.31 (t, J = 128.6 Hz,
H]− Calcd for C9H15NO6P2: 294.0296, found: 294.0302.
δ
= 17.46;
P), 6.99
δ
H
H
δ
= 18.20, 51.89,
C
P), 125.86, 126.99, 128.19, 129.96, 136.50, 138.60; HRMS (TOF MS ESI−): [M
−
N-(3-Methylbenzylamino)methylenebisphosphonic acid (5b) was obtained as a white solid (36% yield);
◦
1
m.p. 248–250 C; 31P-NMR (243.12 MHz, D2O + NaOD, ppm):
D2O + NaOD, ppm): = 2.24 (s, 3H, C 3), 2.65 (t, 1H, J = 17.2 Hz, C
J = 7.0 Hz), 7.14–7.25 (3H, m, Ar); 13C-NMR (151.02 MHz, D2O + NaOD, ppm):
(t, J = 126.3 Hz,
P), 125.29, 127.49, 128.51, 128.99, 138.47, 140.59; HRMS (TOF MS ESI−): [M
Calcd for C9H15NO6P2: 294.0296, found: 294.0301.
δ
= 17.10; H-NMR (600.58 MHz,
P), 3.85 (s, 2H, C 2), 7.06 (1H, d,
= 20.43, 54.48, 59.31
H]−
δ
H
H
H
δ
C
−
N-(4-Methylbenzylamino)methylenebisphosphonic acid (5c) was obtained as a white solid (28% yield);
◦
1
m.p. 244–246 C; 31P-NMR (243.12 MHz, D2O + NaOD, ppm):
D2O + NaOD, ppm): = 2.04 (s, 3H, C 3), 2.45 (t, 1H, J = 16.8 Hz, C
7.02 (AB system, 4H, J = 7.52 Hz, Ar); 13C-NMR (151.02 MHz, D2O + NaOD, ppm):
59.39 (t, J = 194.1 Hz,
P), 128.50, 129.02, 136.93, 137.56; HRMS (TOF MS ESI−): [M
C9H15NO6P2: 294.0296, found: 294.0301.
δ
= 17.50; H-NMR (600.58 MHz,
P), 3.65 (s, 2H, C 2), 7.01 &
δ
H
H
H
δ
= 20.15, 54.28,
H]− Calcd for
C
−

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N-(3-Methylb◦enzylamino)-3-methylbenzylphosphonic acid (6b) was obtained as a white solid (19% yield);
m.p. 150–157 C decomp.; 31P-NMR (243.12 MHz, D2O + NaOD, ppm): = 15.89; 1H-NMR (600.58 MHz,
δ
D2O + NaOD, ppm):
δ
= 2.19 (3H, s, C
H3), 2.23 (3H, s, CH3), 3.40 & 3.42 (AB system, 2H, J = 13.5 Hz,
C
H
2), 3.57 (d, 1H, J = 15.8 Hz, CH
P), 6.94–6.99 (2H, m, Ar), 7.02 (2H, t, J = 9.25 Hz, Ar), 7.09–7.31 (4H, m,
Ar); 13C-NMR (151.02 MHz, D2O + NaOD, ppm): δ = 20.39, 20.53, 51.22, 51.32, 62.96 (d, J = 134.2 Hz),
125.65, 126.05, 126.88, 127.91, 127.71, 128.51, 128.51, 129.42, 129.81, 137.76, 138.53, 139.44, 140.45; HRMS
(TOF MS ESI−): [M − H]− Calcd for C16H20NO3P: 304.1103, found: 304.1111.
N-(4-Methylbenzylamino)-4-methylbenzylphosphonic acid (6c) was obtained as a white solid (7% yield);
◦
1
m.p. 158–160 C; 31P-NMR (243.12 MHz, D2O + NaOD, ppm):
δ
= 17.51; H-NMR (600.58 MHz,
3), 2.53 & 2.72 (2H, AB system, J = 13.1 Hz,
P), 5.68 & 5.69 (4H, system AB, J = 7.8 Hz, Ar), 5.79 (m, 4H, Ar);
H]− Calcd for C16H20NO3P: 304.1103, found: 304.1118. 13C-NMR was
D2O + NaOD, ppm):
2), 2.93 (1H, d, J = 17.3 Hz, C
HRMS (TOF MS ESI−): [M
δ = 0.78 (3H, s, CH3), 0.82 (3H, s, CH
C
H
H
−
unclear because of the low solubility of 6c.
N-(2-Chlorobenzylamino)-2-chlorobenzylphosphonic acid (6d) was obtained as a white solid (24% yield);
m.p. 223–225 ◦C (lit. mp = 205–207 ◦C [32]); 31P-NMR (243.12 MHz, D2O + NaOD, ppm):
δ
= 15.26;
= 3.51 & 3.52 (2H, AB system, J = 30.0 Hz, CH2), 4.23
(1H, d, J = 19.3 Hz, CHP), 7.07–7.14 (4H, m, Ar), 7.21 (1H, t, J = 6.9 Hz, Ar), 7.22–7.27 (2H, m, Ar), 7.59
(1H, d, J = 8.1 Hz); 13C-NMR (151.02 MHz, D2O + NaOD, ppm):
= 49.31, 39.41, 58.4 (d, J = 133.0 Hz,
1H-NMR (600.58 MHz, D2O + NaOD, ppm):
δ
δ
CP), 126.78, 127.16, 128.84, 129.05, 129.39, 129.91 129.94, 130.96, 133.57, 134.50, 136.65, 138.32; HRMS
(TOF MS ESI−): [M − H]− Calcd for C14H14Cl2NO3P: 344.0010, 345.9983 found: 344.0022, 346.0074.
N-(3-Chlorobenzylamino)-3-chlorobenzylphosphonic acid (6e) was obtained as a white solid (31% yield); m.p.
196–199 ◦C decomp.; 31P-NMR (243.12 MHz, D2O + NaOD, ppm):
δ
= 15.11; 1H-NMR (600.58 MHz,
D2O, ppm):
δ = 3.31 & 3.34 (2H, AB system, J = 13.6 Hz, CH2), 3.44 (1H, d, J = 17.9 Hz, CHP), 6.91 (s,
1H, Ar), 6.95 (s, 1H, Ar), 7.04–7.07 (m, 5H, Ar), 7.16 (s, 1H, Ar); 13C-NMR (151.02 MHz, D2O + NaOD,
ppm):
δ = 51.26, 51.35, 62.99 (d, J = 133.0 Hz, CP), 126.20, 127.04, 127.35, 128.59, 128.61, 128.64, 129.31,
129.91, 133.00, 133.44, 141.42, 142.91; HRMS (TOF MS ESI−): [M
344.0010, 345.9983 found: 344.0016, 345.9978.
−
H]− Calcd for C14H14Cl2NO3P:
N-(4-Chlorobe◦nzylamino)-4-chlorobenzylphosphonic acid (6f) was obtained as a white solid (24% yield);
m.p. 262–263 C. 31P-NMR (243.12 MHz, DMSO-d6, ppm):
δ
= 10.32; 1H-NMR (600.58 MHz, DMSO-d6,
ppm):
δ = 6.03 & 6.07 (2H, system AB, J = 13.93 Hz), 4.13 (1H, d, J = 14.391 Hz), 7.39–7.49 (8H, m); HRMS
(TOF MS ESI−): [M
−
H]− C14H14Cl2NO3P required 344.0010, 345.9983 found 344.0005, 345.9965.
13C-NMR was unclear because of the low solubility of 6f.
N-(3-Bromobenzylamino)methylenephosphonic acid (5h) was obtained as a white solid (88% yield); m.p.
230–232 ◦C decomp. 31P-NMR (243.12 MHz, D2O + NaOD, ppm):
δ
= 18.55; 1H-NMR (600.58 MHz,
D2O + NaOD, ppm):
δ = 2.12 (t, 1H, J = 16.82 Hz, CHP), 3.39 (s, 2H, CH2), 6.79 (1H, t, J = 7.7 Hz,
Ar), 6.88 (1H, d, J = 7.7 Hz, Ar), 6.94 (1H, d, J = 7.7 Hz, Ar), 7.08 (1H, s, Ar); 13C-NMR (100.61 MHz,
D2O + NaOD, ppm):
δ = 53.65, 58.74 (t, J = 118.9 Hz, CP2), 121.00, 127.05, 129.58, 130.33, 130.94, 142.91;
HRMS (TOF MS ESI−): [M − H]− Calcd for C8H12BrNO6P2: 359.9226, found: 359.9227.
N-(2-Thiophenylamino)-thiophen-2-ylmethylphosphonic acid (
4
) was obtained as a white solid (54% yield);
m.p. 215–216 C decomp. 31P-NMR (243.12 MHz, D2O + NaOD, ppm): = 14.41; 1H-NMR
(600.58 MHz, D2O + NaOD, ppm): = 3.78 & 3.85 (AB system, 2H J = 14.02 Hz, C 2), 6.00 (1H,
d, J = 11.53 Hz, C
P ), 6.90–6.96 (4H, m, Ar), 7.27 (2H, bs, Ar); 13C-NMR (100.53 MHz, D2O + NaOD,
ppm):
◦
δ
δ
H
H
δ
= 45.45, 57.93 (d, J = 137.4 Hz, CHP), 124.45, 125.35, 126.45, 126.52, 126.53, 126.63, 127.21,
142.28; HRMS (TOF MS ES−): [M − H+], Calcd for C10H12NO3PS2 287.9918, found 287.9912.
Aminomethylenebisphosphonic acid (
α
3) was obtained as a white solid (49% yield in reaction with
-methylbenzylamine, or 85% yield in reaction with p-aminobenzylamine); mp 258-260◦C decomp.
◦
(lit. mp = 250–265 C decomp. [31, δ = 18.12;
33]); 31P-NMR (243.12 MHz, D2O + NaOD, ppm):

Molecules 2017, 22, 450
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1H-NMR (600.58 MHz, D2O + NaOD, ppm):
δ
= 2.63 (1H, t, J = 17.9 Hz, C
H
P), 13C-NMR (151.02 MHz,
D2O + NaOD, ppm):
δ
= 50.80 (t, J = 127.9 Hz,
C
P); HRMS (TOF MS ESI−): [M
−
H]− Calcd for
CH7NO6P2: 189.9670, found:189.9660.
N-(1-Phenylmeth-1-ylamino)methylenebisphosphonic acid (
m.p. 264
266 ◦C decomp. (lit. mp. = 251–253 ◦C [31]); 31P-NMR (161.98 MHz, D2O + NaOD, ppm):
= 9.37; 1H-NMR (399.78 MHz, D2O + NaOD, ppm):
= 1.12 (d, 3H, J = 6.60 Hz, C 3), 2.55 (t, 1H,
J = 16.76 Hz, C P2), 4.47–4.53 (q, 1H, J1 = 6.47 Hz, J2 = 18.27 Hz, C NH), 7.15–7.21 (m, 1H, Ar),
7.27–7.38 (m, 4H, Ar); 13C-NMR (151.02 MHz, D2O + NaOD, ppm):
= 21.40, 55.62 (dd, J1 = 123.32 Hz,
J2 = 123.26 Hz,
P2), 56.64 (bs) ,127.30, 127.59, 128.77, 143.00; HRMS (TOF MS ES+): [M + H]+ Calcd
8) was obtained as a white solid; (13% yield);
−
δ
δ
H
H
H
δ
C
for C9H15NO6P2: 296.0453, found: 296.0464.
N-(3-Amino)benzylaminomethylenebisphosphonic acid (9b) was obtained as a green solid; (24% yield);
◦
1
m.p. 240–242 C; 31P-NMR (243.12 MHz, D2O + NaOD, ppm):
δ
= 17.18; H-NMR (500.14 MHz,
D2O + NaOD, ppm):
δ = 2.56 (t, 1H, J = 19.08 Hz, CHP2), 3.72 (s, 2H, CH2), 6.45 (bs, Ar), 6.67 (1H,
d, J = 6.90 Hz, Ar), 6.72 (1H, d, J = 6.40 Hz, Ar), 7.04 (1H, t, J = 8.15 Hz, Ar); HRMS (TOF MS ESI−):
[M − H]− Calcd for C8H14N2O6P2: 295.0249, found: 294.9994.
◦
1
N,N’-Dibenzylmethanol (14) was obtained as a white solid; (62% yield); m.p. 125 C; H-NMR
(600.58 MHz, CDCl3, ppm):
6H, Ar), 7.31–7.34 (m, 2H), 8.12 (s, 1H, O
δ = 4.38 (d, 4H, J = 14.71 Hz, CH2), 4.82 (s, 1H, CHNH), 7.25–7.29 (m,
H
); HRMS (TOF MS ESI−): [M + H]+ Calcd for C15H18N2O2:
243.1497, found: 243.1175.
1
O-Acethyl-N,N’-dibenzylmethanol was obtained as a orange oil; (89% yield); H-NMR (600.58 MHz,
CDCl3, ppm):
δ = 2.37 (s, 4H, CH2), 4.88 (s, 1H, CH3), 7.23–7.27 (m, 4H, Ar), 7.29–7.35 (m, 6H, Ar) 9.26
(s, 1H, CHNH); HRMS (TOF MS ESI−): [M + H]+ Calcd for C17H20N2O2: 285.1603, found: 285.1612.
All the spectral data see supplementary information.
4. Conclusions
The analysis of the general mechanism of three component condensation of
aminomethylenebisphosphonate formation was carried out using structurally variable benzylamines
as the nucleophilic substrate. The amines was proven to be a valuable tool to follow appearance
of various intermediates, non-phosphorus and phosphorus containing. They were isolated, their
structures were identified and characterized which in turn allowed the suggestion the overall reaction
route. The synthetic pathway leads to aminomethylenebisphosphonates via a series of imine and
phosphonate products. The aforementioned factor can effect, however, with some difficulties in the
final product separation. Since the three-component reaction usually yields a complex mixture of
products that are difficult to separate, the resulting esters are not isolated. Instead, the crude reaction
mixture is hydrolysed, yielding the bisphosphonic acid, which is subsequently isolated.
The three-component reaction between amines, triethyl orthoformate and diethyl phosphite is the
most commonly used route for the preparation of N-substituted aminomethylenebisphosphonic
acids. Surprisingly, benzylamines, depending on the reaction conditions, afforded the desired
bisphosphonates or aminophosphonates or mixtures of the two. The studies presented above indicate
that this is a complex and unpredictable reaction, which is strongly dependent on the applied conditions
(ratio of reagents, presence of oxygen and temperature). However, manipulating the molar ratio of the
reagents and the presence of oxygen can be used to control the course of the reaction.
Addition of greater excess of diethyl phosphite and increasing the reaction time caused visible
enhancement of the final product yield while
2 disappeared immediately. However, the obvious
conclusion that the increase in the yield of target bisphosphonate should be achieved by increase
of molar ratio of diethyl phosphate used is not so apparent when considering that this usually
causes substantial difficulties in separation of pure final products. Formation of compound 6d was
unambiguously shown by X-ray studies.

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Supplementary Materials: 1H-NMR, 13C-NMR, 31P-NMR, IR and MS spectra of representative compounds are
available on line.
Acknowledgments: Support from the National Science Centre, Poland (Grant 2011/01/B/ST5/00843), is
gratefully acknowledged. P.M. thanks to the National Science Centre, Poland (Grant UMO-2015/17/N/ST5/02006)
for their financial support of this work. Linguistic correction was financed by Wrocław Center of Biotechnology,
program, and the Leading National Research Center (KNOW) for years 2014-2018.
Author Contributions: P.M. performed synthetic studies and interpreted NMR spectra; I.T.-T. obtained and
analysed the crystal structure of compound 6d; P.K. interpreted the obtained results and wrote the paper; E.C.
performed the synthetic studies, conceived and designed the experiments and wrote the paper.
Conflicts of Interest: The authors declare no conflicts of interest. The funding sponsors had no role in the design
of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the
decision to publish the results.
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©
2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).